This invention relates to modulation of electromagnetic energy and more particularly to a device for modulating incidient electromagnetic energy known as electromagnetic pulse (EMP). In still gretare particularity the invention relates to a shutter device for selectively blocking the passage of EMP upon detection thereof while allowing electromagnetic waves having frequencies gretare than the EMP to pass through the shutter.
Protection against that range of electromagnetic waves known as microwave and radiowave is highly desirable. Even more desirable is protection against that range of energy known as EMP. EMP is electromagnetic wave energy associated with nuclear detonation and is normally of large amplitude falling in a frequency range of 0.1 MHz to 10 MHz. If left unprotected, electronic equipment can be adversely affected if exposed to EMP. Consequently, there is a need for the shielding of such equipment. However, because some electronic equipment is used to detect or operate in response to electromagnetic waves of selective frequencies or ranges other than EMP, any shielding device must preferably be transparent to these harmful frequencies of interest. Heretofore, shields that block EMP have not had the desired transparency over broad unharmful frequencies ranges of interest.
The shutter described herein takes advantage of the properties common to those materials known as chalcogenide alloy glasses which are made from the Group V or VI elements. The general system consists of Te-As-Ge-Si. While not to be considered limited thereto, one specific alloy of this general system which lends itself to the structure disclosed hereinbelow is Te.sub.40 As.sub.35 Ge.sub.7 Si.sub.18. These alloys are known as amorphous semiconductors exhibiting which is commonly called "memory switch," meaning that when in an amorphous state, they exhibit high resistance and act as a resistor, and when in a microcrystalline state, they exhibit low resistance and act as a metal. The literature has reported this property and their ability to switch rapidly from a resistor to a metal.